This invention relates to electromagnetic actuator assemblies and in particular to novel improved linear voice-coil assemblies adapted for reciprocating magnetic transducer means relative to magnetic recording surfaces.
Workers know that computers today commonly employ magnetic disk files for recording and storing data. Disk files have the advantage of facilitating data transfer at randomly selected address locations (tracks) and without need for a "serial seek" as with magnetic tape. Such transducers must be reciprocated very rapidly between selected address locations (tracks) with high precision. This will be recognized as depending on how fast the system can move a transducer between locations; and do so with high positional accuracy between closely-spaced track addresses. This constraint becomes very tricky as track density increases.
Disk file systems commonly mount a transducer head on an arm carried by a block that is supported by a carriage. This carriage is usually mounted on tracks for reciprocation by an associated transducer actuator. This invention is concerned with improving the efficiency of such actuators; and particularly with improving linear "voice-coil" positioners.
Known Positioners:
The actuators commonly used with magnetic disk files are subject to some exacting requirements; for instance, these systems typically involve a stack of several magnetic disks, each with many hundreds of concentric recording tracks spanning a radius of about 7 inches; and a head-carrying arm is typically provided to access each pair of opposing disk surfaces. This arm will typically carry two to ten heads, each to be reciprocated over a (radial) excursion of several inches to position its heads (adjacent a selected track). Thus, it will be appreciated that such applications require a high positioning accuracy together with very fast translation (to minimize access time--a significant portion of which is used for head positioning.
Disk heads must commonly be reciprocated very rapidly between selected address locations (tracks) with high precision. Thus it is critically important for an actuator system to move a transducer very rapidly between data locations; and to do so with high positional accuracy between closely-spaced track addresses. This constraint becomes ever more burdensome as track density increases--as is presently the case.
That is, such a positioner must move its transducer heads very rapidly so that the associated computer can process data as fast as possible--computer time being so expensive that any significant delay over an extended period can inflate costs enormously. That is, the "transition time", during which heads are moved from track to track, is "dead time" insofar as data processing is concerned.
Now, the present trend is toward ever higher track density with increased storage capacity and decreased access time. Of course, as track density rises, closer control over the actuator mechanism is necessary to position transducer heads accurately over any selected track, lest signals be recorded, or read, with too much distortion, and without proper amplitude control, etc.
Thus, computer manufacturers typically set specifications that call for such inter-track movements within no more than a few milliseconds. Such high speed translation is most demanding on actuators, it postulates a powerful motor of relatively low mass (including carriage weight) and low translational friction. Another requirement for such head positioners is that they exhibit a relatively long stroke, (several inches) in order to minimize the number of heads required per disk.
The prior art disclosed many such positioner devices, including some intended for use in magnetic disk memory systems: e.g., see U.S. Pat. Nos. 3,135,880; 3,314,057; 3,619,673; 3,922,720; 4,001,889; 3,544,980; 3,646,536; 3,665,433; 3,666,977; 3,827,081; and 3,922,718 among others.
Voice Coil Motors:
Workers in the art are familiar with prior art magnetic actuators especially those adapted for reciprocating magnetic transducers relative to magnetic disk surfaces or the like. Such an actuator is the well known voice coil motor VCM or moving coil actuator arrangement shown in FIGS. 1A and 1B. This structure will be recognized as comprising an E-shaped magnet structure M including a central core M.sub.c along which a moving coil C is adapted to be movably mounted. Thus, working flux (see phantom magnetic flux line F) circulating through magnet M traverses the indicated gap g.sub.p, between the pole pieces P and the core M.sub.c, and is intercepted by coil C. When the coil is energized with a prescribed electric current and cuts a certain flux (prescribed flux density B and current i in coil of length L yields certain force F (F=BLI), it will be induced to move as indicated by the arrow. The direction of motion will depend on the polarity of the current relative to the flux, as known in the art.
The "voice coil" motor (VCM) comprises a solenoid like those used to drive an audio speaker. In disk drives, magnetic read/write heads are commonly carried by a carriage driven by a VC motor including a mobile electric coil positioned in a magnetic field and fed by a current of selected intensity and polarity. This magnetic field is typically established by permanent magnet means disposed about the movable coil.
Such a VC linear positioner can exhibit certain disadvantages--for example: undesirably large mass and associated excess power requirements; and drive and control circuitry which is unduly-complicated. That is, such actuators typically involve a relatively heavy carriage; accordingly a lot of inertia must be overcome each time the carriage is accelerated from rest. This acceleration must be maximized to minimize access time. Thus, a great burden is placed upon the power requirements to the voice coil to provide the necessary high acceleration. Such VC actuators are not particularly efficient in converting electrical power either; also they typically require relatively complicated drive and control circuitry to effect the requisite precise positioning despite high acceleration. Further, a VC motor is not sufficiently "linear", i.e., its coil impedance commonly varies with position and thus its force/excursion curve is relatively non-linear. This invention is intended to improve the efficiency and performance of such VC positioners, making them more "linear" in a "dual coil" array.
FIG. 1 represents a conventional moving coil magnetic actuator (VCM) very schematically shown (see also Fujitsu Scientific and Technical Journal June 1972, page 60 and following). Here a moving coil (armature) C will be understood as mounted upon a movable bobbin adapted to reciprocate along the core portion M.sub.c of an E-shaped magnetic circuit M, the circuit also including opposing poles P connected by yoke section Y. Such reciprocation will be responsive to electric current through coil C as is well known in the art.
Here the permanent magnet source of magnetic flux will be recognized as a cylindrical, or semo-cylindrical, shell P with its inner core M.sub.c to be encircled by the moving solenoid coil C. Coil C will be recognized as conventionally translated along core M.sub.c when energized with current (due to inductive interaction with the magnetic flux--see arrows emanating between core M.sub.c and peripheral magnet parts). Force arrows T.sub.F, T.sub.R indicate the resultant reciprocal translation forces so developed (forward, reverse)--the force direction being determined by direction of current through coil C, as well known in the art.
The magnetic flux field set-up by coil current will flow mainly through the "path of least reluctance" (as indicated by flux loops F through the magnet).
Workers are aware that, since the flux return path traverses the cross-section of core M.sub.c, then in certain instances actuator efficiency and the upper limit of operation will be affected by "flux saturation" at this relatively narrow section--whereby an increment in coil current fails to produce a proportionate significant increase in actuator force. One might even say that such incremental current and flux is "wasted". Flux may also be deemed "wasted" insofar as the flux return path traverses yoke portion Y (an "open loop" flux) rather than moving through the "working gap" between coil C and (the inner facing surfaces of) poles P (in a "closed-loop").
In accordance with one salient aspect of the present invention, such a transducer positioner is formed to include a second moving coil. According to this feature, this second moving coil (and associated magnet means) is intended to provide a low leakage "balanced" flux path, one which is more efficient than the prior art ("single coil/single magnet" configurations, which tend to describe a conventional "unbalanced", high leakage flux path). That is, this second coil is provided, along with a companion pole piece, and is so-wound and so-excited as to complete the overall flux gap in a "closed loop" mode. Such "dual coil" actuators advantageously use the second coil, etc., as a "working return" for flux, also this facilitates reducing the magnet mass needed and enhance efficiency.
Workers in the art will be given to understand that such a "closed loop", "balanced" flux path is considerably more efficient than prior art "unbalanced" flux modes, especially as regards leakage; a "balanced" flux also tends to improve "linearity" (i.e., make coil impedance more constant as a function of coil position, giving a more linear force/distance curve for the overall device). Such an improved actuator assembly will be seen as better balanced magnetically and, because of its inherently improved linearity, will no longer critically depend upon magnet thickness (except for achieving a higher flux).
As seen hereafter, it will be readily apparent to workers how such a "dual-coil armature" provides a moving coil structure of improved linearity. Such an improved armature will be seen to give superior performance, e.g., as a disk head positioner with "balanced" flux as compared with the conventional VC positioner.
In accordance with another salient feature, such dual coil positioners are taught in operative combination with a disk drive arrangement.
Thus, one object of this invention is to provide the mentioned and other features and advantages. Another object is to teach the use of such "dual coil VC actuators" in transducer assemblies, especially as adapted for positioning heads in a disk drive. Another object is to provide head actuators for disk drives exhibiting better linearity and yet a further object is to teach the advantageous use of such transducer actuators in disk drive assemblies.